In 3rd Generation Partnership Project (3GPP) Release 11, it has been proved that the CoMP technology can significantly improve system performance. In the CoMP technology, a plurality of cells (or base stations corresponding to the cells) including a serving cell and neighbor cells of the serving cell transmit/receive signals to/from users (interchangeably used with user equipments (UEs) hereinafter) in a coordinated manner in order to reduce inter-cell interference and improve received signal qualities of the base stations/users, and thereby improving a throughput of a communication system. To this end, it is necessary to perform coordination among the plurality of cells.
On the other hand, a CoMP system in a centralized radio access network (C-RAN) architecture has been proposed. FIG. 1 shows a schematic diagram of such a system. As shown in FIG. 1, a central control device and a plurality of remote radio heads (RRH) (three RRHs are shown exemplarily in FIG. 1) connected with the central control device form a base station of the CoMP system, each RRH forms a cell, and the cells constituting a cooperating set. There may be one or more UEs in each cell, including a CoMP UE (e.g., a UE located in a center of the cell) and a non-CoMP UE (e.g., a UE located on an edge of the cell).
In the CoMP system shown in FIG. 1, currently a short-term-information-based centralized scheduling (ST-CS) method is mainly adopted to perform coordination among the respective cells in the cooperating set and schedule users in the respective cells. FIG. 2 schematically shows a first ST-CS method. As shown in FIG. 2, the UE measures a reference signal reception power (RSRP) (or a reference signal reception quality (RSRQ)) transmitted by the base station of the cell and a channel state (i.e., an instant channel state or a short-term channel state) of a radio channel between the UE and the base station in step S201, and reports the RSRP (or the RSRQ) and instant channel state information (CSI) indicating the channel state to the base station in step S202. In step S203, the base station transmits the RSRP (or the RSRQ) and the instant CSI to the central control device. In step S204, the central control device performs joint scheduling based on the above information transmitted by the base station to determine a muting state of the cell on each sub-band, and performs scheduling of the users (including allocation of resources to the users and selection of modulation and coding scheme (MCS) levels for the users) on each sub-band for the cell. In step S205, the central control device notifies each cell of the muting states of the cell on the respective sub-bands and a user scheduling result for the cell. In step S206, the base station operates based on the notified muting states and the user scheduling result. In step S207, the base station notifies the UE of measurement configuration information for instructing the UE to measure the CSI.
FIG. 3 schematically shows a second ST-CS method, in which steps S301 to S303 and step S307 are the same as steps S201 to S203 and S207 respectively. In step S304, the central control device performs joint scheduling to determine a muting state of each cell on each sub-band, and notifies each cell of the muting state of the cell on each sub-band in step S305. In step S306, the base station performs scheduling of users independently (including allocation of resources to the users and selection of MCS levels) based on the notified muting state.
A system performance gain obtained by using the above ST-CS methods is closely related to a transmission delay on a line between the base station and the central control device (i.e., a backhaul line). In the ST-CS method shown in FIG. 2, if the transmission delay on the line between the base station and the central control device is too large, the instant CSI information transmitted in step S203 will become outdated information when reaching the central control device, so that the result of the joint scheduling performed based on the instant CSI information in step S204 does not match the channel state at that time any more; furthermore, since the scheduling of the users is performed in the central control device and then the scheduling result is notified to the base station, if the transmission delay on the line between the base station and the central control device is too large, the scheduling result will also become outdated scheduling result when reaching the base station, so that the scheduling result does not match the channel state at that time, either. In the ST-CS method shown in FIG. 3, there is a problem in step S303 which is the same as that in the first method; in addition, although the scheduling of the users is not performed in the central control device any more, which avoids the problem that the transmission delay causes the outdated scheduling result, it is impossible to obtain a good scheduling result because states of neighbor cells are not taken into consideration when the scheduling of the users is performed in the base station.
Therefore, there is a need for a new cell coordination method, which can mitigate or avoid an impact of the transmission delay of the line between the base station and the central control device on the cell coordination result and the user scheduling result.